1. Field of the Invention
This invention relates to a metal tube and its production method. To be more specific, this invention relates to a small diameter metal tube whose inner surface is smooth and which can be used for a pin, injection needle, connector, electron gun for TV liquid crystal, and the like and its production method.
2. Prior Art
Metal tubes of small diameter such as those having, for example, an outer diameter of up to 1.3 mm and used for a medical pin, injection needle, connector, electron gun for TV, or the like are typically produced by curling a metal thin plate having a thickness of up to 0.2 mm simultaneously with the drawing, welding the abutting edges of the thin plate just before its entrance into a drawing die, drawing the welded member through the drawing die to form a tube having an outer diameter of about 4 to 6 mm, and repeating the drawing process to thereby produce a tube product having a desired outer diameter. FIG. 3 shows typical process of drawing. In FIG. 3, a metal tube 1 which has been formed to an outer diameter of about 4 to 6 mm is drawn through a die 2 having a die bore of smaller cross section to thereby reduce the outer diameter and produce a tube whose cross section is identical with the bore of the die and which has an outer diameter of for example up to 1.3 mm. In the drawing, a plug 3 for defining the inner diameter is inserted in the tube 1 to thereby prevent the occurrence of creases on the inner surface of the tube 1 during the drawing process.
However, when the diameter of the tube 1 becomes reduced after repeated drawings, insertion of the plug 3 in the tube 1 becomes impossible, and such drawing with no plug 3 is inevitable. When the tube 1 is drawn with no plug 3 inserted in the tube 1, the inner surface of the tube 1 becomes creased and the inner surface will suffer from an increased surface roughness which results in the increase in the resistance of a fluid passing through the tube 1. Increase in the surface roughness also results in the increase in the surface area which invites adhesion of dirt and foreign matters. This is a serious problem when the tube is used for a medical application where hygiene is of serious interest. In spite of such situation, there has so far been no small diameter tube which is free from the problem of the rough inner surface.
On the other hand, demand for tubes of smaller diameter exists actually. It is desirable for a person who has a routine work of insertion of an injection needle into the skin, such as a patient suffering from diabetes who must inject insulin on a regular basis to have as little pain as possible when the needle is inserted. The pain caused by the insertion of the needle is associated with the magnitude of the resistance in its insertion. It has been sought to develop an injection needle having a reduced resistance in its insertion, in other words, an injection needle of smaller diameter.